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Full text of "Text Book Of Mechanical Engineering"

"i

i!                       960                              Appendix IV.

2-75 x number of cells, The time of charging must be 12 hours
when the cells are first filled, and may even be 30 or 40 with
some makes of cells. Plates are of various sizes and areas, so
that the charging rate rnay vary from 4 up to 15 amperes per
plate. As soon as the cells are fully charged, the circuit is to be
broken first, and the engine slowed down afterwards. Diagrams
shewing variation of voltage during charge and discharge are
shewn in Fig. 897, from which it will be seen that in charging
the voltage gradually rises from i '9 to 2 '4., while at the same time
the specific gravity of the liquid increases from 1-17 to 1-25, which
is a further test of charge completion. If the charge is carried
far enough, small gas bubbles fill the cell as they rise to the sur-
face, and this effect is called c milking.' Cells refusing to milk
must be removed and examined. In discharging, the E.M. F.
falls rapidly at first to 2 volts, and then gradually to 1*9 volts,
after which no more current must be talcen, there being only one-
quarter of the original energy now left

The charging and discharging times should be approximately
equal, for although rapid discharge does sometimes take place,
the plates not only deteriorate thereby, but a lower efficiency is
maintained, due to heat losses caused by resistance. Also the
most economical charging rate is one-half of the maximum allow-
able. It is important that the battery should be kept as fully
charged as convenient—up to milky at least once per week.
.The discharge must never be complete, nor must the battery
remain idle for any length of time, especially when low in charge;
neither must the normal rates of charge and discharge be ex-
ceeded. ' The liquid may be replenished with pure water as it
evaporates.

The capacity of the battery is measured in

\     i             f average discnarce, )

Ampere a ours = «         anmkres         I  x

And also in

Watt hours = { averf/e ) x j aver^e ) x (t0^ h°urs of
(    volts   J      \ amperes J      V   discharge

Which may also be given in

-r-r -r, T             Watt hours
H.P. hours «=-------------